The functional linkage between morphogenetic signaling and activation, or repression of gene expression during development is provided by transcription factors. It is now clearly established that none of the transcription factors participating in lung development and regulation of gene expression is exclusively lung-specific. Therefore, specificity must be the function of precise interactions amongst non-specific molecules. Two key transcription factors that are not exclusively lung-specific, but figure importantly in lung gene regulation are NKX2.1 and HNF3. We propose that: Hypothesis: Combinatorial protein-protein interactions between NKX2.1 and HNF3 underlie, at least in part, the molecular basis of lung-specific gene regulation. The following Specific Aims will test the above hypothesis: Specific Aim 1: To characterize physical and functional protein-protein interactions between NKX2.1 and HNF3 in regulation of SpB transcription. Specific Aim 2: To determine the mechanisms by which SMAD3 disrupts NKX2.1-HNF3 interactions. Specific Aim 3: To determine the in vivo functional significance of NKX2.1-HNF3 interactions. The results from the proposed studies will undoubtedly have significant functional implications for combinatorial protein-protein interactions as the molecular basis of tissue and cell-type specificity of gene regulation during lung development. [unreadable] [unreadable]